Phase-locked loops (PLLs) are devices that have been used to recover timing information in a wide variety of applications ranging from wireless receivers to memory devices. For example, PLLs may be used with optical data storage systems to extract clock signals, sometimes referred to as the “wobble”, spatially embedded in optical media. When the spinning speed of an optical medium changes—or the radial position of a laser head changes relative to the optical medium—so does the wobble frequency produced by a device reading the optical media. PLLs allow an optical reading device to track changes in the wobble regardless of changes in spinning speed and head position. By locking on to the wobble, a PLL can adjust its output clock frequency so that information may be reliably read from and written to the optical medium.
For a Compact Disc (CD), the nominal bit length is 300 nm. For a writable Digital Video Disc (DVD+), the nominal bit length is 130 nm. An optical read/write device appropriately locked on to the wobble should ideally write bits of exactly the nominal bit length. Those differences between the actual bit lengths of information written onto a disc and the nominal bit length are often referred to as “jitter.” Generally, jitter may cause a host of problems for optical storage systems. For example, if large jitter is present when a device is writing data to an optical disc, the irregular bit information imprinted in the disc may cause difficulty when later retrieving the imprinted information. Accordingly, it is generally advantageous for PLLs in optical read/write devices to reliably track the wobble of an optical disc and to minimize jitter when writing to the optical disc.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.